The present invention generally relates to control systems for controlling the supply of a predetermined quantity of fluid for the purpose of supplying a preset quantity of fluid, and more particularly to a control system for controlling the supply of a predetermined quantity of fluid so as to accurately supply a preset quantity of fluid without introducing an oversupply of fluid.
Conventionally, in most apparatuses for supplying a predetermined quantity of fluid, an integrated value of a flow quantity which is measured in a flowmeter, is supplied to a preset counter. This preset counter generates a predetermined quantity signal when the integrated value of the measured flow quantity coincides with a value which has been preset in the preset counter. The apparatus is designed to close a valve responsive to this predetermined quantity signal. However, the quantity of fluid which flows from the time when the valve begins to close and the time when the valve actually closes completely, should originally be not supplied. This quantity of fluid which should originally be not supplied, is the so-called oversupply quantity.
Accordingly, there was an apparatus which employed a two-step valve closing system for closing the valve, in order to reduce the above oversupply quantity of fluid. In such an apparatus, the valve which is in a fully open state, is closed by a certain amount when the supplied quantity of fluid reaches a value which is close to a predetermined quantity, to continue the supply of fluid at a small flow quantity. The partly closed valve is closed completely when the supplied quantity of fluid reaches the predetermined quantity. These two-step operations improve the accuracy of the apparatus. However, a valve driving device having a complex construction, was required to close the valve in two steps as described above. Therefore, the apparatus as a whole became complex. Moreover, even when the valve was closed in two steps, the oversupply quantity of fluid could not be eliminated completely. In other words, there was a limit in improving the accuracy of the apparatus.
Hence, an apparatus was proposed in a U.S. patent application Ser. No. 298,878 filed Sept. 2, 1981 entitled "Apparatus for Supplying Fluid of Preset Quantity", now U.S. Pat. No. 4,442,953, in which the assignee is the same as the assignee of the present application. This proposed apparatus comprises a fluid supplying pump provided in a fluid supplying pipe arrangement, a motor for driving the fluid supplying pump, a meter for metering the fluid flowing in the fluid supplying pipe arrangement, and a control circuit for detecting that a supplied quantity of fluid measured by the meter has reached a quantity smaller than a preset fluid supplying quantity by an estimated oversupply quantity of fluid, and for stopping the motor from being driven. The estimated oversupply quantity of fluid is set to a quantity which is equal to a quantity of fluid supplied by the fluid supplying pump after the motor is stopped from being driven and rotates due to inertia.
However, because the oversupply quantity itself is dependent on the flow speed of the fluid which is measured at the time when the supply of current to the motor is cut off, the oversupply quantity will change if the flow speed of the fluid changes while the fluid is being supplied due to a change in the voltage which is applied to the motor or the like. For this reason, the calculation of the oversupply quantity had to be performed constantly while detecting the flow speed, and the construction of the apparatus became complex. Further, when the flow speed changed while the flow speed was being measured or when the flow speed changed after the flow speed was measured, the calculated oversupply quantity no longer assumed an appropriate value. In this case, it was impossible to accurately supply a preset quantity of fluid, and the supply of fluid stopped before the preset quantity of fluid was actually supplied, or the oversupply quantity of fluid continued to be supplied even after the preset quantity of fluid has actually been supplied. In the latter case, the supplied quantity of fluid exceeded the preset quantity, and the tank into which the fluid was supplied could possibly overflow.
In addition, the oversupply quantity which occurs due to the inertia of the pump and the flow or current of the fluid, also changes depending on the length of the fluid supplying passage at the ejecting side of the pump, the arranged state of the fluid supplying passage, or the like. Hence, even when the flow speed is accurately measured, there was a problem in that it required a complex control to accurately control the overflow quantity with respect to the preset value, depending on the bent state of the fluid supplying hose, for example.